In the search for oil and gas bearing formations traversed by a borehole, it is common to make measurements with a logging tool which is moved through the borehole. The type of measurement pertinent to this invention is the intensity of secondary gamma ray radiation developed through neutron irradiation. The measurements are made and recorded as a function of depth and the record produced is called a "log".
It is well known that oil and gas are more likely to be found in commercially recoverable quantities from those earth formations which are relatively porous and permeable than in formations which are more highly compacted or impermeable. It is also known that an oil and gas producing formation may be identified by passing a neutron source through the borehole and measuring the intensity of secondary gamma radiation developed from the neutron irradiation as a function of borehole depth.
In particular, the chlorine nucleus which has a very high thermal neutron capture cross-section, .SIGMA., (more so than that of the nuclei of other rather commonly found elements) is a good indicator of the location of salt water. Thus, salt water filled limestone or sandstone layers in the earth will have a greater macroscopic thermal neutron capture cross-section than will oil saturated layers. When combined with other porosity information, oil or hydrocarbons can be detected by determining the chlorine content of a formation. This measurement has been observed in the past by measuring either chlorine capture gamma rays or the neutron lifetime (the decay constant of the thermal neutron population) in the earth layer being investigated.
The above mentioned salt water detection techniques have proven to be very useful in the past in locating oil and gas bearing earth formations. However, spurious indications sometimes have been produced by this logging technique due to the fact that boron also has a very high thermal neutron capture cross-section and is commonly found in shales. Hence, there is always some ambiguity in determining if chlorine or boron is the primary cause of increases in observed .SIGMA. values. An oil bearing zone containing a small shale volume (and hence boron) will have a higher .SIGMA. value than a clean oil bearing zone, and unless the presence of boron is compensated for, the shaly zone may be misinterpreted as being salt water bearing.
The neutron lifetime or thermal neutron decay time logs which are obtained from the bombardment of the formations with high energy neutrons (14 MeV) represent a measure of the rate of decay of the thermal neutron population. This measurement provides a qualitative indication of the elements in the formation. To determine the rate of thermal neutron population decay at successive fixed times following a neutron burst, electronic gates sample the detector output. The ratio of the detector counts (appropriately corrected for background radiation) is directly related to the macroscopic thermal neutron capture cross-section .SIGMA.. In the detector assembly, the pulse height of the output voltage signal is proportional to the energy of the gamma ray detected.
Boron, when irradiated with thermal neutrons, yields an excited state of lithium which quickly decays to a ground state with the emission of a gamma ray having an energy of about 0.48 MeV. Chlorine, hydrogen, iron, silicon and other formation elements all give off thermal neutron capture gamma rays with energies above 0.48 MeV. The thesis of the present invention is to set a discrimination level for gamma ray energy somewhat above 0.48 MeV to detect thermal neutron capture gamma radiation for substantially all elements except boron.
In the prior art, the above described system uses a cutoff discrimination level of about 2.2 MeV to eliminate the background counts attributable to the neutron activation of the detector crystal and other background radiation effects. Although this level effectively cuts off gamma radiation caused by capture of thermal neutrons by boron, it also cuts off a significant percentage of capture gamma rays from other elements such as calcium, iron and chlorine.